Although it is known that the coke consumption of a blast furnace can be reduced by the use of inexpensive fuels or by the direct introduction of reducing agents into the blast furnace, the blowing of coal into the blast furnace has hitherto been adopted in practice only to a restricted extent. This is due essentially to the fact that the conveyance and distribution of fine-particle coal from a storage container into the inlet openings of the blast furnace, which are generally the usual tuyeres provided anyhow, are accompanied by greater problems than with a fluid medium, for example oil or gas. Thus an incomplete reaction with solid fuels can lead to noticeable disturbances in the operation of the blast furnace, for example owing to the formation of soot which reduces the permeability of the charge toward the reducing gas and is sometimes also present in the top gas. The latter can lead to disturbances in the devices traversed by the top gas.
If inexpensive mineral coal is used in order to achieve the desired savings, a new problem arises. In fact, inexpensive mineral coals have high ash contents which can vary between 15 and 25%, the ash being markedly acidic. When using mineral coals of this type, there is a danger of the massive quantities of acidic ash not moving fast enough into the blast furnace slag and of the resulting inhomogeneous oxide mixtures flowing poorly and disturbing operation of the blast furnace.
It has also been found through practice and experience that, when adopting the technology of coal dust blasting, problems arise as in early as the storage containers. The mineral coal dust can have an average particle size below 100 microns, and exhibit the undesirable property of caking in the storage bunker. Attempts have been made to overcome the last-mentioned problem, among others by fitting mechanical all-round conveyors in the individual storage bunkers and continuously revolving the coal dust. Although this method substantially prevents caking and sticking, the conveyors needed represent a considerable cost factor and obviously have to operate absolutely reliably.